The present invention relates to a device for purifying water by means of reverse osmosis, and more specifically to such a device comprising a filter unit containing an osmotic membrane and having an inlet, a filtrate outlet, and a return conduit through which a relatively large flow of unfiltered water is returned to the inlet and a small flow of reject water is discharged from a reject outlet.
Reverse osmosis is an efficient method of purifying water. It is a problem in this connection, however, that the membrane surface is clogged by the separated substances, particularly when the content of calcium of the water is high, which reduces the result of the filtration. In order to avoid such clogging it is common to use a softening device which is provided upstream of the the membrane filter and is regenerated by some suitable salt.
The consumption of salt for regeneration of softeners increases with an increased use of softened water, for example in dishwashers and washing machines. In many places the percentage of salt of the waste water has increased beyond what would be preferable from an environmental point of view.